Ultrasonic flow measuring method

ABSTRACT

An ultrasonic flow measuring method comprises steps of selecting a section area in a right angle to an ultrasonic transit trajectory line for measuring a flow velocity as a section area necessary for a flow measurement; and multiplying a flow velocity component of a direction corresponding to an ultrasonic transit trajectory, which is directly measured by an ultrasonic wave, by the section area thereby to compute a flow or flowrate, so that a flow measuring error and a measuring error of a section area can be significantly reduced, thereby enhancing the accuracy of the flow measurement.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention is related to an ultrasonic flow measuring technology, and particularly, to an ultrasonic flow measuring method for measuring flow velocities on a plurality of fluid flowing sections and then computing a flow or flowrate, if ultrasonic transducers are mounted on a pipe that had been already arranged on a place.

[0003] 2. Description of the Background

[0004] A general ultrasonic flow measuring method is based on the fundamental technical background as follows: an ultrasonic one channel flowmeter is designed to measure a flow velocity V_(D) on a part of a fluid flow section, for example the inner diameter of a pipe, using an ultrasonic wave and multiply the flow velocity V_(D) by a flow coefficient K along with a fluid section area S to calculate a flow. An ultrasonic multi-channel flow measuring method includes steps of measuring a flow velocity V_(D) and flow velocities on chords divided into a plurality of sections, using an ultrasonic wave, to calculating an average flow velocity V_(S) of a fluid flow section and multiplying V_(S) by a section area to calculate a flow. Another method is known to measure an average horizontal flow velocity at a plurality of water depths in an open sluice in order to compute a flow. Typical ultrasonic flow measuring methods and apparatuses there for are disclosed as follows:

[0005] U.S. Pat. No. 5,531,124 granted on Jul. 2, 1996

[0006] U.S. Pat. No. 4,646,575 granted on Jul. 25, 1987

[0007] U.S. Pat. No. 4,860,593 granted on Aug. 29, 1989

[0008] U.S. Pat. No. 5,780,747 granted on Jul. 14, 1998

[0009] U.S. Pat. No. 4,676,321 granted on Jul. 25, 1996

[0010] Russian Patent No. 2,138,782 granted on Sep. 27, 1999

[0011] The ultrasonic flow measuring methods already known have common properties as follows:

[0012] 1) A flow measuring section is selected to be a section S in a right angle to a direction of a fluid flow. In case of a pipe, a section rectangular to a centerline is selected. 2) Therefore, a flow velocity in a right angle direction to a section to be firstly measured by an ultrasonic wave is calculated. At that time, it is assumed that the direction of the flow velocity is corresponded to a fluid flow direction. 3) An ultrasonic flow velocity measuring method includes a frequency difference method and a phase difference method, but these methods are based on transit time difference method, which has been broadly used.

[0013] A typical transit time difference flow velocity measuring expression is as follows: $\begin{matrix} {V = {{{\frac{L^{2}}{2d}t_{2}} - \frac{t_{1}}{t_{1}t_{2}}} = {\frac{L^{2}}{2d}\quad \frac{\Delta \quad t}{t_{1}t_{2}}}}} & (1) \end{matrix}$

[0014] Wherein, L is an interval distance between paired transducers 1 and 2, d is a projection distance of L in which d=L cos φ, t₁ is a transit time in a flow velocity direction from the paired transducer 1 to the paired transducer 2 and t₂ is a transit time in a direction contrary to a flow velocity from the paired transducer 2 to the paired transducer 1 (referring to FIG. 1). A flow computing expression of an ultrasonic one-channel flow computing method is as follows:

Q=K·V _(D) ·S  (2)

[0015] Wherein, K is a flow coefficient, V_(D) is a flow velocity on a diametric line to be measured by the expression (1) and S is a section area of fluid as defined above, for example an inner section area of a pipe.

[0016] One of flow calculation expressions for an ultrasonic multi-channel flow measuring method is as follows:

Q=V _(S) ·S  (3)

[0017] Wherein, V_(S) is a total average flow velocity on a plurality of chords to be measured by the expression (1).

[0018] An ultrasonic flowmeter has most characteristics as follows: unlike another flowmeter, mounting transducers on a pipe that had been already arranged in a place can perform a flow measurement. Even under the condition that fluid is transported through the pipe, the transducers can be mounted on the pipe through the drilling work thanks to the technology progress. For the characteristics, the ultrasonic flowmeter is very often used.

[0019] Particularly, the ultrasonic multi-channel flow measuring method can measure a flow, exactly, even if a condition that K=constant, for example a distance of a straight portion of a pipe becomes at least 25D and Re>10⁴, is not secured and a flow velocity distribution is not a normal state, or if the inner diameter of the pipe is relatively larger. Therefore, the characteristics enable the ultrasonic flowmeter to be used as a flowmeter for a larger pipe.

[0020]FIG. 2 shows five chords for measuring a flow velocity, but the number of chord can be increased as requested. As shown in FIG. 2, in order that d=L₁·cos φi=const, mounting angles φ_(i) of paired transducers 1 _(i) and 2 _(i) are not equal to each another.

[0021] As represented in the expressions (2) and (3), a flow measuring error δ_(Q) is considered as a sum of a flow velocity measuring error δ_(V) and a section area measuring error δ_(S). The flow measuring error δ_(Q) in the ultrasonic one-channel flow measuring method is as follows:

δ_(Q)=δ_(K)+δ_(VD)+δ_(S)  (4)

[0022] The flow measuring error δ^(Q) in the ultrasonic multi-channel flow measuring method is as follows:

δ_(Q)=δ_(Vi)+δ_(M)+δ_(S)  (5)

[0023] Wherein, δ_(K) is a flow coefficient error, δ_(M) is an error followed by calculating an average flow velocity of a section using a flow velocity V_(i) measured on a plurality of chords, for example an approximate integral error of an expression that $V_{S} = {\frac{1}{2R}{\int_{- R}^{+ R}{{V(r)}\quad {{r}.}}}}$

[0024] In the expressions (4) and (5), the flow measuring error δ_(Q) is determined by the flow velocity measuring error δ_(V) and the section area measuring error δ_(S). Therefore, in order to enhance the accuracy of the flow measuring, the flow velocity measuring error δ_(V) and the section area measuring error δ_(S) are significantly reduced. In the flow velocity measuring expression (1), assuming that the transit time measuring errors includes an accidental error component, a flow velocity measuring error is as follows:

δ=(2δL+δd)+{square root}{square root over (δ² t1+δ² t2+δ² Δt)}=(2δL+δd)+A A={square root}{square root over (δ²t1+δ²t2+δ²Δt)}  (6)

[0025] Wherein, δ_(L) is a measuring error of an interval distance L, and δ_(d) is a measuring error of d, in which L and d are a constant to be inputted into an arithmetic logic processor or microprocessor after being measured. Therefore, the symbols of the δ_(L) and δ_(d) are not changed. In other words, these errors are a fixing error. δ_(t1), δ_(t2) and δ_(Δt) are errors of each of transit times t₁ and t₂, and the error Δt=t₂−t₂.

[0026] As represented in the expression (6), even through t₁ and t₂ are precisely measured under the condition that A is reduced enough to be ignored, if δ_(L) and δ_(d) are relatively larger, the flow velocity measuring error δ_(V) becomes larger. Herein, what the measuring error δ_(L) of L is multiplied by 2 is because of L₂. In case of the pipe, the section area S is calculated by measuring ab inner diameter D as follows: $S = \frac{\pi \quad D^{2}}{4}$

[0027] The calculation error of the section area is as follows:

δS=2δD  (7)

[0028] Wherein, δ_(D) is a measuring error of an inner diameter D.

[0029] Therefore, the measuring errors of geometrical integers or constants L, d, D appear as a flow measuring error as follows:

δ_(Q)=(2δL+δ_(d+)2δD)+A  (8)

[0030] These errors are a fixing error represented as an arithmetical sum with their symbols being known.

[0031] In case of a flowmeter of a flange type, the inner diameter D is measured several times to obtain its average value {overscore (D)}, so δS=2δD is secured to become smaller. But, measuring the interval distance L_(i) between the transducers is not simple. There is a measuring instrument capable of measuring an inner diameter, exactly, but there is not a precise measuring instrument that can directly measure an ultrasonic transit distance L_(i) between the transducers disposed at an angle φ to an axis of a pipe. For it, it is very difficult to secure a small value of δ_(Li) enough to be ignored. A measuring error δ_(d) of the projection distance d=L cos δ calculated by measuring the interval distance L and a mounting angle φ of the transducer is as follows:

δd=δL+δ cos φ  (9)

[0032] Herein, ${{\delta cos}\quad \phi} = {\frac{\cos \left( {\phi \pm \alpha} \right)}{\cos \quad \phi} = {{{\cos \quad \alpha} \mp {\tan \quad {\delta sin\alpha}}} - 1}}$

[0033] Therefore, δ_(d) is as follows:

δdMAX=L+(cos α+tan φ sin α−1)  (10)

[0034] Wherein, α is a absolute error of an angle φ to be measured, for example if δ=45° and α=0.25°, δ cos φ≈0.44% . The geometrical distance measuring error δ_(h) is as follows:

δh=2δL+δd+2δD≈3δL+2δD  (11)

[0035] But, if the transducers are mounted on the pipe that had already been arranged at a place, the inner diameter D of the pipe cannot be measured at firsthand. Furthermore, the inner diameter identified by a pipe manufacturer has a predetermined deviation. If an anti-corrosion layer is coated, its thickness cannot be identified. Due to it, it is common that the absolute error of the inner diameter is approximately 2˜4 mm. If ΔD=4 mm, δD=4×100/600≈0.67% , and the section error δS=2×0.67=1.34%.

[0036] On the other hand, there discloses a method of exactly measuring a transit distance L_(i) using an ultrasonic wave. A sound velocity C in fluid of a pipe is measured by a three-point method, and then the transit time t_(1.2) between paired transducers is measured, so L=C×t_(1.2), which suggests the exact value of L. For example, a method which is disclosed in U.S. Pat. No. 5,531,124 issued on Jul. 2, 1996 comprises steps of measuring the transit time t_(1.2) between paired transducers, inserting one transducer into the pipe by ΔL and again measuring the transit time t_(Δ), thereby measuring a flow velocity on the inner diameter of a pipe. $\begin{matrix} {{t_{1.2} = \frac{L}{C}};{t_{\Delta} = \frac{L - {\Delta \quad L}}{C}};{{t_{1.2} - t_{\Delta}} = \frac{\Delta \quad L}{C}}} & (12) \end{matrix}$

$\begin{matrix} {{{\because C} = \frac{\Delta \quad L}{t_{1.2} - t_{\Delta}}}{L_{i} = {{C\left( t_{1.2} \right)}i}}} & (13) \end{matrix}$

[0037] If the transit time t_(1.2) and the distance ΔL are very precisely measured, the error of L_(i) gets smaller. On the contrary, if the inner diameter is larger, the error of L_(i) obtained by the expressions (12) and (13) may become larger. The reason is as follows: the sound velocity C obtained by the expression (12) is a sound velocity in an interval of ΔL, but it may be not equal to the sound velocity in the interval ΔL. In other words, if a fluid temperature of the interval ΔL away in a predetermined distance from a pipe wall is not corresponded to an average temperature of all intervals L_(i), the sound velocity C obtained by the expression (12) is not the same as a sound velocity CL_(i) in the interval L_(i). If ${{\Delta \quad L} = \frac{L_{i}}{2}},$

[0038] C is equal to C_(Li). But, if the inner diameter of the pipe is larger, the length of the transducer for measuring the sound velocity is extended, because L_(i) becomes larger.

[0039] A main object of the invention is to provide to an ultrasonic flow measuring method for measuring flow velocities on a plurality of fluid flowing sections and then computing a flow or flowrate, if ultrasonic transducers are mounted on a pipe that had been already arranged on a place.

[0040] Another object of the invention is to provide an ultrasonic flow measuring method for significantly reducing an error component of geometrical integers necessary for measuring and calculating a flow velocity and a flowrate.

[0041] Another object of the invention is to provide an ultrasonic flow measuring method for enabling the same mounting angle of each of paired transducers to facilitate the transducers to be mounted on a pipe that was already arranged on a place.

SUMMARY OF THE INVENTION

[0042] According to the invention, an ultrasonic flow measuring method comprises steps of selecting an inner section area Sφ of a pipe cut at an angle δ of 45° as a flow measuring section, in which the inner section area Sφ is an ellipse or oval form, mounting paired transducers at two points having a longer diameter of the inner section area Sφ, mounting a predetermined number of paired transducers along the periphery of the ellipse on both sides by the center of the longer diameter, measuring flow velocities on a plurality of chords of the ellipse using an ultrasonic wave, computing an average flow velocity of the section area Sφ and multiplying the average flow velocity by the section area Sφ to measuring the flow or flowrate, in which the longer diameter of the section area Sφ is subject to being measured using the ultrasonic wave.

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] The invention now will be described in detail in reference to the accompanying drawings, in which:

[0044]FIG. 1 is a view illustrating an ultrasonic flow velocity measuring method according to a prior art;

[0045]FIGS. 2A and 2B are views illustrating a configuration of ultrasonic multi-channel flow velocity measuring cords of a prior art;

[0046]FIG. 3 is a schematically view illustrating a method of selecting a flow measuring section according to the invention;

[0047]FIG. 4 is a schematically view illustrating a method of measuring a flow by mounting a plurality of a pair of transducers according to the invention;

[0048]FIG. 5 is a schematically cross-sectional view illustrating a sound velocity measuring apparatus for exactly measuring a distance between paired transducers according to the invention; and, FIGS. 6A and 6B are schematically views illustrating a method of measuring a flow in a sluice, in which FIG. 6A is a planar view illustrating the mounting state of paired transducers for measuring a horizontal average flow velocity at a plurality of water depths, and FIG. 6B is a cross-sectional view of FIG. 6A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0049] A flow or flowrate Q is to multiply a section area S in a right angle to a direction of a flow velocity by a section average flow velocity V_(S). If the flow velocity direction of fluid flowing in a pipe is corresponded to the centerline of the pipe, the direction of the flow velocity V_(L) to be measured in a first step using an ultrasonic wave is as follows:

V _(L) =V cos φ  (14)

[0050] Therefore, a value q multiplying a section S_(e⊥) at a right angle to the flow velocity V_(L) by the flow velocity V_(L) is the same as that of multiplying the flow velocity V to the section area S.

q=V _(L) ·S _(e⊥) =V·S  (15)

[0051] Such like a relationship is represented in FIG. 3. In FIG. 3, the section area S is as follows: $S = \frac{\pi \quad D^{2}}{4}$

[0052] A section area S_(e⊥) of an ellipse is as follows: $S_{e\bot} = {\frac{\pi}{4}L_{e}D}$

[0053] Wherein, L_(e) is a longer diameter of the ellipse S_(e⊥), and D is a shorter diameter that is equal to the inner diameter of the pipe. The longer diameter L_(e) is as follows: $L_{e} = \frac{D}{\cos \quad \phi}$

[0054] Therefore, S_(e⊥) is as follows: $\begin{matrix} {S_{e\bot} = {\frac{\pi \quad D^{2}}{4\cos \quad \phi} = \frac{S}{\cos \quad \phi}}} & (16) \end{matrix}$

[0055] The expression (16) is substituted into the expression (15) and then into the expression (14) instead of V_(L). Whereby, the following expression is obtained. $q = {{V_{L}S_{e\bot}} = {{{V\cos \phi}\frac{S}{\cos \quad \phi}} = {V \cdot S}}}$

[0056] If φ=45°, assuming that an area of the ellipse is S_(e) and paired transducers 1 and 2 are mounted on the apex points forming the longer diameter of the ellipse, an interval distance L between the paired transducers is constituted as a longer diameter of the ellipse S_(e). If φ≠45°, the ellipse area S_(e⊥) subject to being selected is as follows: $\begin{matrix} {S_{e\bot} = {{S\quad {\phi tan\phi}} = {{\frac{\pi}{4} \cdot D \cdot L \cdot \tan}\quad \phi}}} & (17) \end{matrix}$

[0057] Wherein, L is an interval distance between paired transducers 1 and 2 disposed at both apex points of a longer diameter of an ellipse, and L_(e)=L tan φ which is a longer diameter.

[0058] If φ=45°, for example tan 45°=1, S_(e⊥)=Sφ, the longer diameter of which is L.

[0059] The direction of a flow velocity V_(L) measured by an ultrasonic transit time difference method corresponds to that of a line L connecting the paired transducers 1 and 2 to each other, and the flow velocity V_(L) is as follows: $\begin{matrix} {V_{L} = {{\frac{L}{2}t_{2}} - \frac{t_{1}}{t_{1}t_{2}}}} & (18) \end{matrix}$

[0060] The expression (18) is derived as follows: $t_{1} = {\frac{L}{C + {V\quad \cos \quad \phi}} = \frac{L}{C + V_{L}}}$ $t_{2} = {\frac{L}{C - {V\quad \cos \quad \phi}} = \frac{L}{C - V_{L}}}$

[0061] Therefore, from the above expression the relative expressions are established as follows: $\begin{matrix} {{C + V_{L}} = \frac{L}{t_{1}}} & (a) \\ {{C - V_{L}} = \frac{L}{t_{2}}} & (b) \end{matrix}$

[0062] That is, $\frac{L}{t_{1}} - \frac{L}{t_{2}}$

[0063] is obtained as follows: ${2V_{L}} = {{{L\quad \frac{t_{2} - t_{1}}{t_{1}t_{2}}}\quad\therefore V_{L}} = {\frac{L}{2}\frac{t_{2} - t_{1}}{t_{1}t_{2}}}}$

[0064] Therefore, the expression (18) is established. Herein, it is said that the transit time method is dependant on the sound velocity C², but it is substantially a wrong thing.

[0065] As shown in FIG. 4, if φ=45°, a plurality of paired transducers are mounted along the periphery of an ellipse having a longer diameter of L, a flow velocity V_(Li) is measured on a plurality of chords of a ellipse section area S_(φ) to compute an average flow velocity V_(Sφ) and then the average flow velocity V_(Sφ) is multiplied by the ellipse section area S_(φ) to obtain a flow of flowrate Q m²/s of fluid passing through a pipe. And, the mounting angles of paired transducers 1 _(i) and 2 _(i) are equal to each another. In other words, the same angle φ of 45° must be secured. Therefore, it is easy to install the paired transducers on the pipe compared with a prior art, because the paired transducers are mounted at the same angle φ of 45° along the cutting angle of the pipe. Like this, it is not necessary to calculate the flow velocity V_(L) into a flow velocity direction component corresponded to a centerline of the pipe.

[0066] In the expression (18), a measuring error δ_(VL) of the flow velocity V_(L) is as follows:

δVL=δL{square root}{square root over (δ² _(t1)+δ² _(t2)+δ²tΔ)}=δL+A  (19)

[0067] The expression (19) is compared with the measuring error expression (5) of the flow velocity V based on the prior are as follows:

δV−δVL=(2δL+δd+A)−(δL+A)=δL+δd  (20)

[0068] In other words, the measuring error becomes smaller under the same condition by δ_(L)+δ_(d). Then, the comparison between measuring errors δ_(S) and S_(Sφ) of the section areas are as follows:

δS=2δD

δSφ=δL+δD

[0069] If δ_(L)<<δ_(D), the measuring error of the ellipse section area S_(φ) is reduced two times as small as the conventional one. The effect become significant in case that δ_(D) is larger because it is not possible to directly measure a pipe inner diameter, when the paired transducers are mounted on the pipe that had been already arranged on a place. For example, if δ_(D)=1%, δ_(S)=2 δ_(D)=2.0. If δ_(L) can be ignored by measuring Le or L, exactly, δ_(Sφ)=δ_(D)=1%.

[0070] A method of measuring L, exactly, is as follows:

[0071] When the paired transducers are mounted on the pipe, a valve is previously mounted. As shown in FIG. 5, a container 3 is fuilly filled with fluid by opening the valve of the pipe. A sound velocity in fluid of the container is measured. To the end of it, a supporting bar 6 includes transducers 4 and 5, which are mounted to be spaced at a predetermined interval from each other thereon. The supporting bar 6 is disposed at a predetermined depth in the container 3. First, the transducers 4 and 5 are placed to have an interval distance l₁ there between to measure an ultrasonic transit time tl1. Then, the transducer 5 is moved by an additional distance l₁, so that l₂=2l₁. At that time, a transit time tl2 is measured. Therefore, the following expression is established. $t_{l1} = {\frac{l_{1}}{C} + \tau + {\overset{\_}{\Delta}t}}$ $t_{l2} = {\frac{2l_{2}}{C} + \tau + {\overset{\_}{\Delta}t}}$

[0072] Wherein, τ is a delay time of an electrical signal in a transit time measuring circuit, and Δt is a fixing absolute error of the transit time measuring circuit.

[0073] The sound velocity C is obtained using t_(l1) and t_(l2) as follows: $\begin{matrix} {C = \frac{l_{1}}{t_{l2} - t_{l1}}} & (21) \end{matrix}$

[0074] If accidental errors of t_(l1) and t_(l2) are {tilde over (Δ)}_(t), an error Δtl=tl2−tl1 is as follows: $\begin{matrix} {\delta_{\Delta \quad {tl}} = {\frac{\sqrt{2\quad {\overset{\sim}{\Delta}}_{t}^{2}}}{t_{l2} - t_{l1}} = \frac{1.4 \times {\overset{\sim}{\Delta}}_{t} \times C}{l_{1}}}} & (22) \end{matrix}$

[0075] Therefore, it is ease to secure {tilde over (Δ)}_(t) that is equal to 2.2⁻⁹.S. If l₁=0.5m and C=1500 m/s, the measuring error is as follows: $\delta_{\Delta \quad {tl}} = {{\frac{1.4 \times 2 \times 10^{- 9} \times 1500}{0.5} \times 100} = {8.4 \times 10^{- 3}\%}}$

[0076] In case that l₁=0.5 m, it is easy to measure l₁within the error of ±0.5 mm. For example, $\delta_{l1} = {\frac{0.05 \times 100}{500} = {0.01\%}}$

[0077] Therefore, a measuring error δ_(C) of the sound velocity C is as follows:

δC=δl+δΔtl=0.01+8.4×10⁻³=0.01%

[0078] As a transit time t_(i) between the paired transducers is measured using the sound velocity C, an interval distance L_(i) between the paired transducers can be exactly measured using L_(l)C×t₁.

[0079] If a part of a pipe for mounting paired transducers of an ultrasonic flowmeter is made as a flange type, one side of the pipe is clogged not to leak fluid there from. The pipe is vertically put up to be fully filled with fluid, and then the sound measuring device as shown in FIG. 5 is disposed in the pipe to obtain the interval distance L_(i) by measuring the sound velocity. Such like sound velocity measuring method is used for a sound velocity measurement regarded as a three point measuring method.

[0080] According to the invention, a flow measuring method comprises steps of measuring a flow velocity component V_(L) of fluid corresponding to an ultrasonic transit trajectory L, which is at a certain angle to a flow velocity direction of fluid in order to measure a flow velocity V of fluid flowing through a pipe by using an ultrasonic wave, and multiplying a value of the flow velocity component by a fluid section S_(e⊥), thereby to compute a flow or flowrate. Even if the direction of the flow velocity V is not identical to the centerline of the pipe, for example the flowrate is measured at a point near an elbow portion of the pipe, a larger error doesn't occur.

[0081] A flow measuring method of the invention is used in an open sluice, which is shown in FIGS. 6A and 6B. Paired transducers l_(i) and 2 _(i) are disposed at a plurality of depths on a line forming an angle φ of 45° to the centerline of the open sluice. A section area Sφ is used as a flow measuring section, which is calculated by measuring a plurality of depths along a line of an interval distance L_(i). Only, under the condition that the section of the open sluice is evenly distributed in an interval of d that is equal to L cos φ, the flowrate can be very exactly measured without identifying an angle of a skew flow, even if a flow velocity of a skew flow component is developed due to a curved portion of upper or lower stream of the open sluice.

[0082] Another effect is as follows: if the paired transducers are mounted along dotted lines I and II according to a conventional method as shown in FIG. 6, the mounting angles of the paired transducers become different from each another. For it, the mounting angles of the paired transducers should be adjusted, but it is very difficult to perform the adjusting work, because the paired transducers are immersed in fluid. On the contrary, according to the invention, the paired transducers are disposed on the straight line of the interval distance, L, and their arranging angles are also identical to each another. Only, the invention is requested to secure the angle φ by adjusting a paired transducers' supporting bar.

[0083] As described above, the invention is explained centering on a method of measuring a flow velocity V_(L) using the transit time difference method, but the invention has the same effect in using a phase difference method. 

We claim:
 1. An ultrasonic flow measuring method for measuring a flow velocity at a plurality of chords on a section of fluid using an ultrasonic wave and multiplying the flow velocity measured by the fluid section thereby to compute a flow comprising steps of: selecting a section area Sφ cutting the fluid flow at an angle φ Of 45° to a direction of the flow velocity as a flow measuring section; measuring the flow velocity on a plurality of chords dividing the section area Sφ using an ultrasonic wave to compute the flow velocity at the section area Sφ; and, multiplying the section flow velocity by the section area Sφ to compute the flow, in which the flow computing section area is Sφ tan φ, if the angle φ is not secured to be 45°.
 2. The ultrasonic flow measuring method as claimed in claim 1, in which: the ultrasonic flow measuring method furthermore comprises steps of selecting the section area Sφ of an ellipse form, mounting one of paired transducers for measuring the flow velocity on the apex of the ellipse having a longer diameter, and mounting another paired transducers on left and right sides by the reference of the apex to form multi-channels for the ultrasonic flow velocity measurement, if the ultrasonic flow measuring method is adapted to a pipe.
 3. The ultrasonic flow measuring method as claimed in claim 1, in which: the ultrasonic flow measuring method furthermore comprises step of disposing paired transducers for the horizontal average flow velocity measurement at a plurality of depths alone a circumferential line contacting with fluid of a section area Sφ corresponding to the gradient surfaces of both banks of the open sluice, if the ultrasonic flow measuring method is adapted to an open sluice.
 4. The ultrasonic flow measuring method as claimed in claim 2, in which: the computing step of the section area Sφ comprises steps of measuring a sound velocity by using an interval distance L between paired transducers mounted on two apexes forming the longer diameter of the ellipse section area Sφ and a three point method and multiplying the sound velocity by an ultrasonic transit time between the paired transducers to obtain the longer diameter of the ellipse. 